AN APPARATUS FOR CONTROLLING THE HEATING OF FOODSTUFFS

Abstract: When preparing food by heating it, it is the temperature of the contents of a cooking pot which is important, not the temperature on the outside of the bottom. Known thermostats only measure the latter or simulate the heat transfer. The invention utilizes the breakdown as a function of temperature of a zero frequency magnetic field after switching off an electric current to control the power supply to a cooking pot. The magnetic field may be established by means of suitable elements in the pot itself or in a resonant circuit with temperature dependent elements placed in the contents of the pot. A detector coil measures the velocity of the breakdown and signal processing eliminates noise signals in the measurement.

Description: An apparatus for controlling the heating of food¬ stuffs. The invention relates to an apparatus for controlling the electrical power supply to the contents of a cooking pot on a hob. Constructions are known which are based on measuring the temperature of the pot bottom by means of a sensor plate which is pressed against the bottom of the pot by means of a spring, whereupon the sensor in a way which is selected from many possibilities controls the supply of energy until the preselected temperature has been reached, whereupon there is a switching between energy supply and disconnection in order that the temperature is kept constant. This may e.g. be in the neighbourhood of the boiling point for the content. This construction has the advantage for in particular cooking plates of the traditional type, i.e. solid and spiral, that it may be positioned in the centre of the cooking plate and is hence out of the way. The disadvantage by this could be that the temperature measured may be influenced by the cooking plate itself, but in practice there is a greater resistance to heat conduction between cooking plate and sensor than there is between the sensor and the pot bottom against which it rests through spring pressure, which all diminishes the harmful influence. For cooking plates of modern types, e.g. ceramic, halogen or induction which display a plane hob surface it is a decided disadvantage that wells have to be formed into which the sensors are to be fitted, and in practice such constructions are not used with these types of hob. Furthermore components are known which in conjunction with the direct energy control constitute a model for the heat transmission between the cooking plate and the content in a pot. In its simplest form these have been known as a "Simmerstat" (reg. trade mark) which have a thermostatic wire with a well-defined heat transfer in conjunction with a knob which intermittently switches the full power in and out. However, this must be manually adjusted for each pot and cooking plate, and hence it of no consequence that the electromechanical construction has been replaced by electronic solutions. Furthermore thermometric solutions are known in which a remote thermometer is connected to the hob by means of a cable for transmitting the signals from the thermometer to the electronic control unit. This provides a control of energy supply which is ideal from a temperature point of view, since it is only the thermal time constant because of the heat supply and the pot which influence the speed of control, but in practical use on a hob it is a construction which is not suitable. This is in particular caused by the fact that it is impossible to use a tight fitting lid, that the thermometer sensor may transfer a taste from pot to pot, and that it may be difficult to keep track of a number of remote thermometers, if there is one thermometer per cooking plate. It is the purpose of the invention to provide a construction for control of the electric power supply to the content in a pot which avoids the described disadvantages and which permits a large flexibility with respect to which cooking plates are to be supplied with such a possbility. This is obtained in a construction according to the invention which is particular in that the measurement of the temperature is performed by creating a magnetic d.c. field in proximity to the contents of the pot, to switch off the current which creates this field, and to detect the temperature dependent decay of the magnetic field in a subsequent measuring period, and that the result is used to control the heat energy supply to the content of the pot in dependence of the distance from the desired temperature. An advantageous embodiment utilizes the same coil to create the magnetic field and to detect the decay of the magnetic field in a subsequent measuring phase. A further advantageous embodiment utilizes well- conducting parts of a pot as a coil, the energy dispersion of which as a result of the decay of the magnetic field is detected by means of measuring probes placed in or below the surface of the hob. A further advantageous embodiment utilizes a sealed resonant circuit which is put into the content of the pot, comprising a coil and a capacitor, the resistance, capacitance, or self inductance is temperature dependent and whose energy dispersion as a result of the decay of the magnetic d.c. field is detected by means of measuring probes placed in or below the surface of the hob. An advantageous embodiment utilizes several steps in the temperature measuring process, in that a pot with a wellknown content in a calibrating phase during performance of a standard measuring cycle gives a signature for the pot which is stored, whereupon this value is used as a correcting reference in a subsequent use of the same pot for heating pot contents. The invention is based on the recognition that a magnetic field which has been created by means of a direct current in a stationary coil with a fixed and uniform geometry, by the removal of the current will decay as a function of those components which have been influenced by the magnetic field. It is then of minor importance, whether it is the frequency of a damped oscillation that is detected or whether it is the time constant itself of the decay of the field, measured as the induced voltage in a stationary coil. The important for obtaining a control signal for a power supply is that there is an influence on the decay which may be directly related to the temperature of the contents of a pot. In the case of a damped oscillation it will in practice be a resonant circuit which has been energized by the supply of the d.c. field, i.e. it has absorbed energy which is discharged with the frequency of the resonant circuit. In the case of decay of a field it will in practical terms be the self inductance and internal resistance of the pot itself which determines the decay, and it will in that case to the largest degree be the temperature dependent specific resistance which influences the speed with which the pot discharges the stored magnetic energy. It is obvious that in case a broad frequency range is considered, there will be high frequency phenomena of short duration which are caused by the existance of capacitances in connection with the pot, but their influence is so small that they may be regarded as parasites from a measuring point of view. In utilizing the invention the mechanism as such for supplying energy to the contents of the pot is completely irrelevant, but it is obvious that certain advantages may be obtained in connection with intermittent connection of an energy source in that the measurement phase may suitably be synchronised with a phase during which no power is supplied to the content of the pot. The invention will be described in detail in the following with reference to the drawing in which Fig. 1 shows a general arrangement with a cooking plate, a transmitting/receiving coil, and a pot, with the necessary electronics. Fig. 2 shows functions for the decay of the magnetic field which is briefly stored by pot parts, Fig. 3 shows functions for the decay of the magnetic field which is briefly stored by a temperature sensitive unit placed in the contents of the pot, Fig. 4 shows a schematic construction for the measurement of the time constant for decay, Fig. 5 is a schematic representation of the performance of self-calibration. In Fig. 1 is shown a hob top plate 1, preferably of a temperature stable but non-metallic material on which is placed a pot 2 with contents 3. For clarity a fatter line indicates that the pot has a well-defined conductance, in particular near the bottom 4. Directly below the top plate there is fitted a heating element 5 which heats the pot, and with it the contents. Surrounding the heating element concentrically to the pot is fitted an outer coil 6 which may build up a magnetic field through the pot and an inner coil 7 concentric thereto which is used as a detector coil for magnetic field. In the contents of the pot is placed a rounded body 8 which contains a resonant circuit with a coil which may couple with both the stationary coils. The coils 6, 7 and the heating element 5 are coupled to a control unit 9 which generally creates an electric signal in function of the temperature of the pot contents and controls the power supply to the heating element in dependence thereof for obtaining a temperature which is adjusted by acting on the control unit at the input 10. The measurement procedure starts in that a current generator 11 is connected by means of a controlled switch 12 to the outer coil which is impressed with a constant current until a constant magnetic field has been built up which will have occured after a number of time constants determined by the self inductances and capacitances and internal resistance in the coil and pot. Thereupon the current through the coil is switched off and after a short waiting period which allows the dispersion of high frequency phenomena the decay of the induced voltage in the detector coil is measured. Instead of a direct measurement of the time constant it may be advantageous to integrate the signal after the passage of a suitable reference level. The arrangement shown in Fig. 1 may either be used with the mentioned rounded body or with a pot which has a well- defined conductivity, never with both parts simultaneously. In case it is the rounded body that is being measured, either the resonance frequency or the decrement of the oscillation may be measured. It has shown itself to be advantageous to use only one coil for both the building up of a field and for the measurement of the field, in that it is only required that proper timing of the connections to the control unit is made. In Fig. 2 the basic function of the decay of a magnetic field in a measuring phase after a charging phase is shown. At a) is shown the period in which the switch connects the coil to the current source, and at b) the resulting voltage V across the coil 7. After a time ta which is used for active break-down of the field contribution from the coil (either by means of a controlled current through the coil 6 or by means of a controlled current in connection with switching a single coil from building-up to measurement) it will be seen that the induced voltage in the coil from the negative value rises towards 0 with a particular time constant. In that the voltages vary over a very large interval it is advantageous to display the function in a logarithmic scale as at c) where it is also seen that the exponential decay becomes rectilinear. Functions for the temperatures ca. 20 degrees Celsius and ca. 100 degrees Celsius are shown, and it is seen that the voltage decays faster because of the higher resistance at 100 degrees. A lower resistance would allow the current in the pot to run for longer time. Various factors influence the precision of the energy control: The time that the magnetic field has been supplied to the pot and its value, the velocity with which the magnetic field is actively broken down, the placement of the pot with respect to the coil, and obviously interference noise and thermic noise. All these factors show that the most certain measurements would be obtained by integration and by letting the integration periods be equal to a complete period of the mains frequency whereby the noise which is synchronous with the mains frequency is eliminated. It is obvious that modern digital signal processing enables an efficient control of the power with corrections for the various outer influences without the various tricks which are shown here in connection with an analogue signal processing. In the digital signal processing it is important that the window is placed correctly in relation to noise pulses and that the sampling frequency is sufficiently high that the high frequency processes may be eliminated actively and not only by filtering in the anti-aliasing filters. In Fig. 3 is shown the basic function for the oscillation which is obtained when the magnetic field decays when the resonant circuit in the rounded body is used. A measurement of the peak values as a function of time gives the logarithmic decrement and hence the time constant for the decay of the field. In Fig. 4 is shown a circuit for the measurement of the time constant using double integration in an iterative process. The measurement is based on the observation that if the proportion between the integrals is precisely e (the base of natural logarithms) the time shift between the start times of the integrations will be precisely equal to the desired time constant. At the measurement points indicated by the numbers 0- 5 in a) the voltages will be as shown at b) . At c) it is shown how the self-compensating value for the time constant is obtained from the integrals used. The time delay which which is equal to the time constant is obtaind after adjustment of the DELAY-unit shown after performance of a number of integration cycles which only stops when the difference between the integrals is equal to zero. In Fig. 5 is shown how it is possible to let the hob detect which pot out of a number of possible pots is in actual use, in order to obtain a link between the measurements of time constants and the contents temperature they represent. It is hence a question of storing a characteristic for each pot. Thereby the apparatus itself will ensure correct control of the power in order to obtain a given chosen temperature for the contents of a pot. The measurement proper of the time constant is performed in the unit 13 which is controlled by the control unit 9 as is the current generator 11. The measured time constant is used in ordinary running for controlling the power supply by means of the units 15 and 16 because a switch 14 is placed in the ordinary position. In case the switch is connected to the unit 17 this logs calibration data which are composed of data for a time constant and date from the unit 15 which report the supply of power. These data are used for modifying control data from the unit 13 in the unit 18 each time the power for the recognized pot is to be controlled during ordinary running. In case a control function from the control unit 9 is established to the switch 14 a checking measurement may be performed repeated times of the pot in use during the preparation of food. In the embodiment shown it is as a minimum required that an empty pot has been put on the hob at room temperature which gives one point on a characteristic. If one may presume that the user will accept that the calibration phase must be performed with a certain care, a practical process will be to put the given pot with a content of 1 1 of water on the apparatus and perform a boiling by supplying power until the temperature and hence the measured time constant has reached a constant value. With knowledge of the power supplied it is possible to obtain and to store a calibration curve. Such a function is most adapted to be used in a digital implementation of the apparatus. However, it may also be possible to utilize some of the phenomena of higher frequency which consequently are of shorter duration for characterization but the chosen method will be dependent on a skilled balancing of the increased requirement for data collection and digital signal processing. It is obvious that the illustrated embodiment represents only one of the many possible methods. Hence there are different requirements for user participation. On may e.g. require of a suser that a button marked "new pot" be pushed, as it must be considered reasonable to have a "reset" button in case the owner of a set of pots, the characteristics of which have been stored should move from the hob, taking the pots with him.

Claims: P A T E N T C L A I M S 1. An apparatus for controlling the electrical power supply to the contents of a cooking pot on a hob in dependence of the temperature of the contents c h a r a c t e r i s t i c i n that the measurement of the temperature is performed by creating a magnetic d.c. field in proximity to the contents of the pot, to switch off the current which creates this field, and to detect the temperature dependent decay of the magnetic field in a subsequent measuring period, and that the result is used to control the heat energy supply to the content of the pot in dependence of the distance from the desired temperature. 2. An apparatus according to claim 1, c h a r a c t e r i s t i c i n that the building-up of the magnetic field occurs by means of a coil which is supplied with a constant current for a time which is sufficient to obtain a constant field. 3. An apparatus according to claim 1, c h a r a c t e r i s t i c i n that the same coil is used to create the magnetic field and to detect the decay of the magnetic field in a subsequent measuring phase. 4. An apparatus according to any of the preceding claims, c h a r a c t e r i s t i c i n that we11-conducting parts of a pot us utilized as a coil, the energy dispersion of which as a result of the decay of the magnetic field is detected by means of measuring probes placed in or below the surface of the hob. 5. An apparatus according to any of the claim 1-3, c h a r a c t e r i s t i c i n that a sealed resonant circuit is put into the content of the pot, comprising a coil and a capacitor, the resistance, capacitance, or self inductance of which is temperature dependent and whose energy dispersion as a result of the decay of the magnetic d.c. field is detected by means of measuring probes placed in or below the surface of the hob. 6. An apparatus according to any of the preceding claims, c h a r a c t e r i s t i c i n that the measuring probes are coils. 7. An apparatus for the control of the electrical energy supply to the contents of a pot in dependence of the contents of the pot, c h a r a c t e r i s t i c i n that several steps in the temperature measuring process are used, in that a pot with wellknown contents in a calibrating phase during performance of a standard measuring cycle gives a signature for the pot which is stored, whereupon this value is used as a correcting reference in a subsequent use of the same pot for heating pot contents.

Inventors: CHRISTENSEN KELD, JENSEN TORBEN, CHRISTENSEN, KELD, JENSEN, TORBEN
Applicants: CHRISTENSEN KELD, JENSEN TORBEN, VOSS FABRIK ATLAS HUSHOLDNINGS, CHRISTENSEN, KELD, JENSEN, TORBEN, VOSS FABRIK - ATLAS HUSHOLDNINGSAPPARATER A/S
Application year: 1994
Main classification: G05D23
IPC codes: F24C15, G05D23, H05B6